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Collection, Preservation and Display of Old Lawn Mowers

2 stroke engine running erratically

I restored a 1930s 2 stroke Qualcast 16 mower about 8 years ago and it has served me well.  However last year it started over-revving, as if it was drawing in air, and I had to mow the lawn constantly adjusting the throttle lever.  I stripped the engine down again over the winter truing up the crankcase halves and fitting new gaskets and plain bearings only to find that the problem has not gone away.  A classic sae 40 oil has been suggested and I've also used a classic sae 30 which make no difference.  There seem to be 2 adjustments that can be made on the Villiers carburettor which I have adjusted endlessly which alter the running of the engine but do not solve my problem.  Has anyone in the club got experience of one of these mowers who could shine more light on this please?


wristpin Sat, 14/05/2022

I think that the time has come to do a vacuum and pressure tests on the. As well as a suitable combined pressure and vacuum generator such as a Mityvac, you will need to make blanking plates for the inlet and exhaust ports with connection / vacuum connectors. For a one off operation you may be able to borrow a Mityvac or similar.

Mityvac with a selection of home made adaptors.


Series3boy Sun, 15/05/2022


Could the crank shaft be worn? Which might still let air in.

Have you got another carburettor to try on that engine? I have had a similar issue. It was rust from the tank blocking the gauze on the fuel inlet to the carb. If it does the same with a different carb. It would point the issue to air leaks air leaks on the engine?





hillsider Sun, 15/05/2022

Re the over revving problem take a close look at the clamp joint that secures the carburettor to the inlet stub, an air leak at this joint can be a source of erratic running. 

I have seen a carb that formed a crack starting in the corner of the clamp cutout and running towards the carb body easily rectified with a layer of JB Weld and a smooth idle was restored.

Series3boy Mon, 16/05/2022

I was thinking an air leak at this joint could be the issue. Or as you say a crack in the carburettor. If you sprayed something like carb cleaner or WD 40 around that joint when the engine is running you might notice it rev up.

pyneapplechunks Tue, 17/05/2022

I did a leak test on the engine before and after rebuilding it.  It leaked air from both crankshaft bearings before and one after new plain bearings were fitted from Villiers Services.  I rang Villiers Services who told me you can't pressure test a 2 stroke engine.  The crankshaft would have to be a tight fit to stop air leaks and of course it wouldn't then rotate.  With my inexperiencd eye I cannot tell how much of a leak is acceptable but it seemed to me there was little or no play in the crankshaft bearings.  Villiers also went onto say that their classic 2 stroke sae oil 'gums up' in use to form a seal of sorts.  I bought some from them,as I needed some more anyway, but after several hours of use it did not make any difference.  They also said to increase the tension on the throttle spring of the carb. which I also did which altered the running and in conjunction with adjustments to the taper needle.  Ishall fit anothr carb. to see if that makes a difference

DJD Fri, 20/05/2022

Some of the old Villiers 2 stroke engines used felt seals on the crank, others nothing, due to very tight tolerances maybe of the long bronze bushings, it seems odd nowadays to even contemplate such a set up, but they had nothing else to use in those days except leather etc, natural materials all.

pyneapplechunks Sat, 21/05/2022

I tried a different carb. but this had no effect then I fitted a spare engine with very slight play in the crankshaft bearings and this also had the effect of over-revving.  I guess the problem must be air being drawn in along the crankshaft so tolerances must be very tight on these plain bearings.  Perhaps the crankshaft could be ground with new bearings to suit.

wristpin Sun, 22/05/2022

I wonder whether you could improvise a couple of oil soaked felt seals just to test the theory ? Crank grinding and undersized bearing bushes ( if available) will be a whole new ball game. May be worth talking to Villiers Services to find out whether they would undertake the work .


hillsider Sun, 22/05/2022

Re the suspected air leak around the crank bearings try dripping some oil around the crank/ bearing housing as the engine is running and listen for a change in the engine note if access is difficult try smearing grease around the crankshaft and the end of the bearing to form a temporary seal, this method may be usefull behind the magneto where access is difficult.

Just as an aside vaccuum testing is possible on two stroke engines and is frequently used on chain saw engines, agreed they do have shaft seals fitted.

Series3boy Sun, 22/05/2022

Yes I wonder if they could be build up with weld and turned down to fit your new bearings. 


DJD Sun, 22/05/2022

Some old Seagull outboard two strokes used no oil seals at all, I've just remembered, the mix was very strong, about 15 to one or even thicker, if I remember correctly, the reason being an oily inner crankcase and 'oil return' grooves in shaft, if memory serves me correctly?

pyneapplechunks Tue, 24/05/2022

Arrows where felt washers will go.The mention of felt seals has got me thinking and I think there is a good chance that they will work.  The space between the magneto and the end of the bearing is about 3/16" and similarly on the sprocket side.  I have ordered a piece of felt 1/4" thick, to allow for a little compression, to make some felt washers.  See photo. with arrows.

pyneapplechunks Fri, 27/05/2022

Yes, I've soaked them in some thick oil and fitted them.  All I have to do now is wait for the grass to grow and will report back on how the engine behaves.

pyneapplechunks Wed, 01/06/2022

After just cutting the grass the engine ran well for the first 10 mins. or so but after 3/4 hour the problem was well and truly back.  I removed the felt washers and made new ones this time with a 5/8" hole so it would be a tight fit over the 3/4" shaft and impregnated them with Castrol heavy grease.  The felt I would say was the right thickness as it compressed by about 1/16".

  The mower is now ready to test again next week.  I was wondering if the heavy grease will just melt away with the heat and so lose any seal that it may make.

pyneapplechunks Thu, 02/06/2022

Photo. shows the felt seals that were fitted.  They have now been replaced by new felt seals impregnated with Castrol heavy grease and the hole diameter reduced to 5/8" to make them a tight fitFelt seals. on the 3/4" shaft.

pyneapplechunks Tue, 07/06/2022

The tighter felt seals with heavy grease haven't made any difference to the running of the machine.  Any further suggestions, apart from scrapping it and buying another mower,  would be welcome.

wristpin Tue, 07/06/2022

Let’s go back over the symptoms.

It starts ok and runs ok for the first ten minutes?

Then shows the symptoms of running weak, speeding up uncontrollably  ?

When  fully cooled down will it repeat this pattern without any further intervention ? 

It does this regardless of the level of fuel in the tank ?

Is or is not affected by gradient or load?

Both your engines exhibit the same symptoms ?

pyneapplechunks Wed, 08/06/2022

The felt seals I fitted do not seem to have made any difference to both engines.  After probaly more like 5 mins. or less they start to show symptons of running lean regardless of how much fuel is in the tank.  The revs. really start up when I reach the end of a line of cutting and I start turning the mower round.  It is not so much of a problem if the engines are just ticking over or moving very slowly.